Process for producing largely nonferrous metal chalcogenides or arsenides with a grain size distribution in the nanometer range

ABSTRACT

The invention relates to a process for producing largely nonferrous metal chalcogenides or arsenides which can be easily carried out in terms of process engineering in an environmentally-friendly manner. This is because in a single process step the desired, largely nonferrous metal chalcogenide or arsenide is obtained from a nonferrous metal powder by reaction with reactants selected from the group sulfur, selenium, tellurium, and arsenic by grinding in a reaction mill in an inert atmosphere.

The invention relates to a process for producing largely nonferrous metal chalcogenides or arsenides with a grain size distribution in the nanometer range. The concept “largely nonferrous” is used for the purposes of the invention since in an industrial production process and the materials used impurities with iron can never be entirely excluded. For example, an iron content of at most 50 ppm is cited.

The elements sulfur, selenium and tellurium form, besides oxygen, the 6th main group of the periodic system and are called chalcogens (metallogens). Their compounds with metals in the earth's crust of course form naturally occurring ore minerals. For technical applications such as lubricants and friction materials, electrode materials for rechargeable high-performance batteries and semiconductor materials, crystalline metal chalcogenides and arsenides which have a defined chemical composition which is not ordinarily present in naturally occurring ore minerals are required. The metal chalcogenides and arsenides which are required for technical applications are therefore produced synthetically from defined parent materials.

Thus metal chalcogenides can be precipitated from metal salt and alkali chalcogenide salt solutions, however amorphous products also being obtained which must be converted into a crystalline form by heat treatment. This method is complex since expensive metal salts are used as the parent products. Furthermore, the impurities which occur must be separated and disposed of.

In addition, producing metal chalcogenides by grinding of reaction mixtures is known, steel balls being used as the grinding media. When using steel balls however large amounts of iron pass into the material being ground with the formation of FeS or complex sulfides as the grinding media are consumed. When grinding thermally reacted reaction products with steel balls, likewise iron passes into the material to be ground.

Therefore DE 198 15 992 A1 proposes a process in which a solid lubricant based on tin sulfide can be produced by heating a tin powder-sulfur mixture in a muffle furnace at 200-1500° C. by thermal reaction. Since sulfur has a melting point of 119.7° C. and tin has a melting point of 232° C., at a reaction temperature of only 200° C. all the sulfur is present in liquid form. Furthermore, the reaction proceeds exothermally, so that as the temperature continues to rise the sulfur used begins to boil and thus toxic sulfur vapors are formed. These sulfur vapors also contain highly toxic sulfur dioxide which is likewise environmentally harmful and therefore must be bound by suitable process techniques. After the end of the reaction the initially liquid mass must be cooled and the crystalline reaction product obtained must be ground to the desired grain size. Therefore, in the process according to DE 198 51 992 A1 several time-consuming and labor-intensive process steps are necessary to obtain the final product. Furthermore, when the final product which contains still free sulfur is ground, in conventional grinding assemblies iron sulfide can be formed in part by taking up iron.

The object of this invention is therefore to make available an industrially applicable, environmentally friendly process in which metal chalcogenides or arsenides are obtained in largely nonferrous form with grain sizes in the nanometer range.

As claimed in the invention, a process of the initially mentioned type is proposed which is characterized in that the nonferrous metal powder with reactants selected from the group sulfur, selenium, tellurium and arsenic is ground in a reaction mill in an inert atmosphere.

Advantageous embodiments of the process as claimed in the invention are disclosed according to the dependent claims.

The invention is detailed below using possible embodiments.

EXAMPLE 1 Production of Bismuth Sulfide (Bi₂S₃)

In an eccentrically driven vibratory mill, 66 kg of a mixture of 53.6 kg bismuth power with an average grain size of d₅₀=15 μm and 12.4 kg sulfur powder are ground for 120 minutes using 3720 kg of hard metal balls with a diameter of 35 mm. The rpm of the vibratory mill is 960 min⁻¹, the vibratory circle diameter is 20 mm. The degree of filling with the grinding media is 80%. A crystalline reaction product is obtained which has an average agglomerate grain size of 16.4 μm. The size of the individual crystals is in the nanometer range.

EXAMPLE 2 Production of Tin(II) Sulfide (SnS)

In an eccentrically driven vibratory mill, 14.592 kg of a reaction mixture consisting of 11.52 kg air-vaporized tin powder with an average grain size of d₅₀=40 μm and 3.072 kg sulfur powder are ground for 60 minutes using 930 kg of hard metal balls with a diameter of 15 mm. A crystalline reaction product is obtained which has an average agglomerate grain size of 18.7 μm. During grinding, at intervals of 15 minutes samples are taken and are studied by means of x-ray diffractometry. With increasing length of grinding the proportion of SnS increases until the metallic tin and the sulfur have reacted almost completely. The iron content of the product is less than 50 ppm.

EXAMPLE 3 Production of Tin(IV) Sulfide (SnS₂)

In a laboratory eccentrically driven vibratory mill, 552 g of a reaction mixture consisting of 360 g air-vaporized tin powder with an average grain size of d₅₀=10 μm and of 192 kg sulfur powder were ground for 30 minutes using 36 kg of hard metal balls with a diameter of 15 mm. A crystalline reaction product of tin sulfide SnS₂ with an average grain size of 12.1 μm is obtained.

In these exemplary embodiments metal chalcogenides or arsenides are obtained by mechanochemical reaction of the solid reaction components at temperatures of roughly 100° C. in only one single process step. To do this, a mixture of the reactants metal powder and chalcogenide powder or arsenic powder in a precomputed stoichiometric ratio is ground under a protective gas (nitrogen or argon) in a suitable grinding assembly, for example an eccentrically driven vibratory mill. The temperature during grinding rises due to the added mechanical energy and due to the reaction heat which is being released, but the melting point is not reached so that a liquid phase is not formed. In the grinding process the grinding media act as a heat-absorbing buffer due to their high heat capacity, in continuous operation corresponding cooling of the grinding assembly however being necessary.

Furthermore, to achieve a certain degree of reaction the conventional grinding time can be reduced by using especially fine-grain initial powders.

Likewise dispersed phases such as graphite or metal oxide which are intimately ground into the crystal structure which forms during the grinding process can be added to the reaction mixture.

By use of nonferrous hard metals—such as tungsten carbide or zirconium oxide or a nonferrous pseudoalloy of tungsten carbide with a metallic binding matrix, for example cobalt which does not react with the free sulfur of the reaction mixture or the already formed sulfides—as claimed in the invention, it is possible to carry out the grinding process without absorbing iron and the associated wear of the mill armoring and grinding media. The synthesis of highly pure metal chalcogenides or arsenides for electrochemical or photovoltaic applications is thus possible using pure parent materials. No significant oxide content and environmentally harmful, gaseous emission could be detected either. 

1-14. (canceled)
 15. Process for producing largely nonferrous metal chalcogenides or arsenides with a grain size distribution in the nanometer range, characterized in that the nonferrous metal powder with reactants selected from the group sulfur, selenium, tellurium and arsenic is ground in a reaction mill in an inert atmosphere.
 16. Process as claimed in claim 15, wherein the reaction mill is a multimodule eccentrically driven vibratory mill.
 17. Process as claimed in claim 15, wherein the mill lining and the grinding media consist of nonferrous hard metal and/or a nonferrous pseudoalloy of tungsten carbide with a metallic binding matrix.
 18. Process as claimed in claim 17, wherein the metallic binding matrix consists of cobalt.
 19. Process as claimed in claim 17, wherein the nonferrous hard metals are tungsten carbide and/or zirconium oxide.
 20. Process as claimed in claim 15, wherein the metal powder and reactants are used in stoichiometric ratios with reference to the metal chalcogenides or arsenides to be produced.
 21. Process as claimed in claim 15, wherein the progress of grinding is monitored by x-ray diffraction analysis.
 22. Process as claimed in claim 15, wherein reaction grinding takes place in batch operation.
 23. Process as claimed in claim 15, wherein elements selected from the group bismuth, tin, copper, indium, gallium, zinc, aluminum, titanium, molybdenum and tungsten are used as the metal powder.
 24. Process as claimed in claim 15, wherein the metal powder is used as a nonferrous metal alloy.
 25. Process as claimed in claim 15, wherein mixtures of two or more pure metals except for iron are used as the metal powder.
 26. Process as claimed in claim 15, wherein other powdered additives such as graphite, carbides, nitrides, oxides or silicates which do not participate in the reaction itself, but are present intimately mixed with the reaction product after grinding, are added to the reaction mixture.
 27. Process as claimed in claim 15, wherein to prevent formation of agglomerates paraffins are added to the reaction mixture.
 28. Process as claimed in claim 15, wherein the metal chalcogenides or arsenides produced are free of iron. 